The present invention relates generally to wireless networks and, more particularly, to measuring and analyzing radio frequency (RF) interference proximate and within a wireless field device mesh network.
Mesh networking is a flexible network architecture that is becoming prevalent in industrial applications. A mesh network includes a cloud of nodes and a gateway computer (gateway) that connects a high-speed bus to the mesh network. Mesh networks avoid many of the limitations of other network topologies by allowing neighboring nodes within the same network to communicate directly with each other, avoiding unnecessary routing of communications to the gateway. A software program known as a network manager, typically running on the gateway, assigns each node multiple communications pathways that are interchanged to compensate for bottlenecks and linkage failures. By allowing neighboring nodes to form communications relays directly to the target node, and by routing around failures or bottlenecks, network response time is improved while minimizing network power usage by minimizing the number of transmissions required to relay communications. Using multiple communication pathways provides path diversity which improves network reliability.
A wireless mesh network is a communication network made up of a plurality of wireless devices (i.e., nodes) organized in a mesh topology. In a true wireless mesh network, which may also be referred to as a self-organizing multi-hop network, each device must be capable of routing messages for itself as well as other devices in the network. The concept of messages hopping from node to node through the network is beneficial because lower power RF radios can be used, and yet the mesh network can span a significant physical area delivering messages from one end to the other. High power radios are not needed in a mesh network, in contrast with point-to-point systems which employ remote devices communicating directly to a centralized base station.
The use of lower power radios is essential for wireless network systems designed for sensor/actuator-based applications, such as a wireless field device mesh network. Many devices in the network must be locally-powered because power utilities, such as 120 VAC utilities or powered data buses, are not located nearby or are not allowed into hazardous locations where instrumentation, sensors, and actuators must be located without incurring great installation expense. “Locally-powered” means powered by a local power source, such as a portable electrochemical source (e.g., long-life batteries or fuel cells) or by a low-power energy-harvesting power source (e.g., vibration, solar cell, or thermo-electric generator). A common characteristic of local power sources is their limited power capacity, either stored, as in the case of a long-life battery, or produced, as in the case of a thermo-electric generator. Often, the economic need for low installation cost drives the need for battery-powered devices communicating as part of a wireless sensor network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a well functioning wireless sensor device. Batteries are expected to last more than five years and preferably last as long as the life of the product.
In order to save power, some wireless field device network protocols limit the amount of traffic any node or device can handle during any period of time by only turning their transceivers ON for limited amounts of time to listen for messages. Thus, to reduce average power, the protocol may allow duty-cycling of the transceivers between ON and OFF states. Some wireless field device network protocols may use a global duty cycle to save power such that the entire network is ON and OFF at the same time. Other protocols, such as Time Division Multiple Access (TDMA) based protocols, may use a local duty cycle where only the communicating pair of nodes that are linked together are scheduled to turn ON and OFF in a synchronized fashion at predetermined times. Typically, the network manager assigns a link to a pair of nodes, as well as a specific time slot for communications, an RF channel to be used by the transceivers, who is to be receiving, and who is to be transmitting, if need be, at that moment in time (e.g., a TDMA with a RF channel hopping protocol, such as WirelessHART®). The network manager synchronizes the duty cycle and assigns multiple communication pathways, coordinating communication between nodes, generating control messages, communications schedules and data queries to suit the situation.
The self-organizing capability of mesh networks to form alternate paths for communicating between devices and between devices and a gateway provides redundant paths for wireless messages. This enhances communication reliability by ensuring that there is at least one alternate path for messages to travel even if another path gets blocked or degrades due to environmental influences or due to RF interference. Nevertheless, even with the robust communication reliability inherent in a mesh network, RF interference from unknown sources can degrade the performance of the network. Using alternate paths to circumvent interference typically results in more hops due to reduced range and energy-wasting re-transmissions to get a message to or from the gateway. If the RF interference is severe enough, all transmissions to and from a node may be blocked for as long as the RF interference persists.
RF interference sources are often intermittent and transient in nature making their detection and identification difficult and time consuming. Detecting and locating sources of RF interference in real time would allow rapid identification and mitigation of the sources, further improving network reliability. Systems have been proposed to monitor interference in wireless communication networks, such as cell phone networks, however such systems are generally unsuitable for wireless field device mesh networks due to the relatively high power requirements of such systems. RF site surveys are expensive since they require specialized RF equipment and specially trained personnel. Even then, the information provided is only a snapshot in time of the true RF environment and may miss important transitory RF interference events. Finally, the data from a site survey quickly becomes stale due to ongoing changes in the surrounding physical plant and in plant infrastructure, as well as changes occurring “outside the plant fence”.